The present invention relates to methods of etching thin film resistor material, and more particularly to a method of cleanly etching NiCr or CrSi to form thin film resistors in semiconductor devices.
Thin film resistors are well known and may comprise a variety of resistive materials, with the chromium-based resistive materials nickel chromium (NiCr) and (CrSi) being common. Thin film resistors may include an electrical interconnect contacting the resistive material for passing a current therethrough. The efficiency and accuracy of the thin film resistor depends upon the condition of the resistive material and the cleanliness of the portion of the resistor contacting the electrical interconnect. Resistive material that is corroded or contaminated (e.g. has highly oxidized chromium) is undesirable because such conditions adversely affect the sheet resistance of the resistor. A "dirty" contact between the resistive material and the electrical interconnect adversely affects the contact resistance of the resistor.
Known methods of manufacturing thin film resistors are considered to be "dirty" because they contaminate and corrode the resistive material and increase the oxidation of the chromium, thereby adversely affecting the sheet resistance of the resistor. Known methods also leave "dirty" contacts between the resistive material and the electrical interconnect, thereby adversely affecting the contact resistance of the resistor.
One of the known methods of manufacturing NiCr thin film resistors includes depositing a photoresist (PR) mask over NiCr film and removing selected portions of the PR mask to expose portions of the NiCr. The exposed NiCr is then directly etched with an enchant that does not affect the PR mask, such as ceric sulfate. Ceric sulfate is known to be a "dirty" enchant which leaves residual ceric oxide contaminants across the wafer surface where the NiCr was removed. However, subsequent processing to remove the ceric oxide has an adverse effect on the exposed Furthermore, the PR mask and the PR strip process also leave residual contaminants, such as carbon and sulfur, on the surface of the exposed NiCr which has an undesirable corrosive effect on the NiCr film. The combined effect of these contaminants increases the oxidation of the Cr within the NiCr film, thereby adversely affecting the sheet resistance of the NiCr resistor and degrading the contact between the NiCr and an electrical interconnect, thereby degrading the performance of the NiCr resistor.
Another known method of producing NiCr thin film resistors uses a PR lift-off process rather than directly etching the NiCr. The lift-off process includes depositing a NiCr film over a patterned PR mask having open areas so that portions of the NiCr film are deposited into the open areas. After the NiCr is deposited, the PR mask is lifted-off to remove the portions of the NiCr not in the open areas. The lift-off process, however, leaves behind unwanted NiCr and PR residue which is difficult to remove and which adversely affects the contact between the NiCr and the electrical interconnect. As in the direct etch method, the lift-off process leaves residual contaminants, such as carbon and sulfur, on the surface of the NiCr. The unwanted contaminants adversely affect the contact resistance between the NiCr and the electrical interconnect. They also have a corrosive effect on the NiCr (e.g. increase the oxidation of the chromium) and thus adversely affect the sheet resistance.
Accordingly, it is an object of the present invention to provide a novel method of etching thin film resistor material that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method of producing uncontaminated thin film resistors.
It is still another object of the present invention to provide a novel method of producing a thin film resistor in which a hard mask is deposited on a chromium-based film and etched with a first etchant that does not affect the film, and the exposed film is thereafter etched with a second etchant that does not affect the hard mask.
It is yet another object of the present invention to provide a novel method of cleanly etching a chromium-based thin film resistor in which portions of the material not to be etched are covered with a hard mask and in which the hard mask is etched with hydrogen peroxide (the first etchant) that does not affect the thin film.
It is a further object of the present invention to provide a novel method of cleanly etching NiCr in which portions of the NiCr not to be etched are covered with a hard mask and in which the NiCr is etched with sulfuric acid heated to greater than 125.degree. C. (the second etchant) that does not affect the hard mask.
It is yet a further object of the present invention to provide a novel method of cleanly etching CrSi in which portions of the CrSi not to be etched are covered with a hard mask and in which the CrSi is etched with a mixture of primarily phosphoric acid (the second etchant) that does not affect the hard mask.
It is still a further object of the present invention to provide a novel method of manufacturing thin film resistors in which electrical interconnects for the resistors are patterned with a PR and in which the PR does not contact and thereby contaminate the thin film resistor.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.